Skills give a bonus from +1 to +3 on any roll depending upon how relevant they are to the situation. If they have none then they give none. Most Bonus are +2.

Lets say a Hero has Forest Survivor. And he is in the Forest and wants to try to track something. This is a broad use of the skill. So he gets a +2.

But say he wants to find some food and shelter for the night, then he will get a +3 to the role. Now lets say he wants to help a group find food and shelter, then it is a +2. This is because his skill is really about finding food and shelter for one and not say six.

Now lets say he finds himself in the grasslands and wants to find Food and Shelter, then the GM might give a +1 (if say the grasslands are healthy). But lets say he wants to try to track in the grasslands… that is a bridge too far. No bonus for his skill.